Abrasive media blasting using aluminum oxide, silicon dioxide and other grit particles is a commonly used processing step in the manufacture of nickel, cobalt and iron base superalloy gas turbine engine hardware, such as turbine blades and vanes, by casting and protectively coating the casting. Examples of processing or manufacturing operations using abrasive grit blasting include a) ceramic shell mold removal from the casting using coarse (e.g. 90 grit or larger) grit particles at high carrier air pressure (e.g. 60-90 psi) , b) surface cleaning of the casting using fine (e.g. 220 grit or smaller) particles at moderate carrier air pressure (e.g. 40-60 psi) prior to aluminizing to form a protective coating, c) roughening a casting surface using fine grit at moderate carrier air pressure prior to platinum or other electroplating operation, or d) roughening a casting surface using large grit at high carrier air pressure prior to overlay coating deposition by plasma or flame spraying or EB-PVD (electron beam-physical vapor deposition). Abrasive grit blasting unfortunately results in grit particles, such as alumina grit particles, becoming embedded as contaminants in the blasted surface of the superalloy component. In general, the severity of surface contamination with embedded grit particles increases with the carrier air pressure and the angle of incidence of the abrasive blast media relative to the casting surface.
Embedded abrasive oxide grit particles in protective diffusion aluminide or MCrAlY overlay coatings can have a detrimental effect on the performance of the coatings. Specifically, embedded grit particles can concentrate stresses (thermal and/or mechanical) applied to the coated engine component, and such stress concentration can result in cracking of the coating and/or the component. In addition, embedded grit particles can adversely affect the adherence of either outwardly grown diffusion aluminide coatings or MCrAlY overlay coatings of the known type (where M is Ni and/or Co and/or Fe), particularly when small grit particles cover a significant fraction of the surface area of the component prior to coating. Because of the undesirable effects of entrapped grit oxide particles, most gas turbine engine manufacturers limit the size and quantity of embedded grit particles permissible in protective coatings. Consequently, processes have been included in manufacture of such components as turbine blades and vanes to control surface contamination from embedded abrasive grit particles. Such processes have included chemical etching of the component surface and fluoride ion cleaning among others.
Grit blasting is inexpensive and widely used in manufacture of such superalloy components as turbine blades and vanes. Frequently, multiple grit blasting operations are incorporated into the production routing of such superalloy components as turbine blades and vanes from shell mold removal to aluminizing. However, in order to reduce manufacturing costs and processing time, surface finishing operations to remove embedded grit particles may be reduced in number to an extent that there can be a chronic problem meeting the aforementioned specifications of gas turbine engine manufacturers with respect to size and quantity of embedded grit particles in protective coatings. Currently, fluoride ion cleaning, ferric chloride etching and ultrasonic cleaning are used to reduce the presence of embedded grit particles at the pre-aluminide coating and/or pre-electroplating stage of manufacture. However, none of these processes is without serious limitations. For example, grit removal using HF/H.sub.2 mixtures at high temperature produces unacceptable alloy depletion in the component and involves considerable expense. Chemical etching of the component using ferric chloride is disadvantageous in that such etching attacks and removes a portion of the alloy surface itself, and so, reduces the component wall thickness. Ultrasonic cleaning is disadvantageous because it only removes some of the embedded particles.
There thus is a need for a process which can be incorporated into the manufacture of superalloy components to remove embedded grit particles effectively without degradation of the component. The present invention has an object to satisfy this need.